(a) Technical Field
The present disclosure relates to an apparatus and a method for measuring the internal ohmic resistance of a fuel cell system, and more particularly, to an apparatus and a method for measuring the internal ohmic resistance of a fuel cell system, using a current interruption method even while the fuel cell system is being operated.
(b) Background Art
A fuel cell system mounted to fuel cell vehicles includes a fuel cell stack in which about tens to hundreds of fuel cells are stacked, a fuel supply system for supplying a fuel (hydrogen gas) to the fuel cell stack, an air supply system including an air blower and a humidifier for supplying oxygen gas in the air which is an oxidizer needed for an electrochemical reaction to the fuel cell stack, and a heat and water management system for controlling an operation temperature and cooling of the fuel cell stack.
When hydrogen gas is supplied to a fuel electrode (anode) of the fuel cell stack and air is supplied to an air electrode (cathode) of the fuel cell stack while the fuel cell system is being operated, an oxidation reaction of the hydrogen gas is conducted in the fuel electrode so that hydrogen ions (protons) and electrons are generated The generated hydrogen ions and electrons are moved to the air electrode through a polyelectrolyte membrane and a bipolar plate of the stack, water is generated in the air electrode through an electrochemical reaction in which the hydrogen ions and the electrons moved from the fuel electrode and oxygen gas of the air participate, and electric energy is generated by a flow of electrons at the same time.
The actual voltage according to the production of the electric energy of the fuel cell is determined as a value obtained by subtracting a loss due to an inner current of the fuel cell stack, a loss due to reaction activation of the stack, a loss due to an ohmic resistance within the stack, and a loss due to a delivery of reactants from an ideal voltage.
While the fuel cell system is operated, the polyelectrolyte membrane constituting a Membrane Electrolyte Assembly (MEA), and the air electrode (cathode) and the fuel electrode (anode) stacked on opposite surfaces thereof tend to deteriorate. Due to this deterioration, performance of the fuel cell is reduced after operation for a period of time. At this time, the internal ohmic resistance of the fuel cell is also varied depending on degree of deterioration of the stack.
The ohmic resistance of the fuel cell corresponds to configurations within the fuel cell stack which serve as a resistor according to Ohm's law, and is known to vary in magnitude depending on water moisture content of the membrane electrolyte assembly of the fuel cell, which includes the polyelectrolyte membrane and the air and fuel electrodes stacked on the opposite surfaces of the polyelectrolyte membrane.
Thus, the degree of deterioration of the fuel cell and the water moisture content within the membrane electrolyte assembly can be indirectly obtained through measurement of the internal ohmic resistance of the fuel cell.
A current interruption method and an alternating current impedance method may be exemplified as a typical method of measuring the internal ohmic resistance of the fuel cell according to the related art.
In the alternating current impedance method, an operational characteristic of the fuel cell is determined by measuring the resistance value of a cell at a specific alternating current frequency. By this method, separate hardware is required for generating an alternating current signal, and a significant amount of time is taken to measure the resistance.
Meanwhile, in the current interruption method, an operational characteristic of the fuel cell is determined through a slope characteristic of a voltage according to a time instantaneously appearing when a current is interrupted. For this, an analysis device is relatively simple, and a small amount of time is taken to measure the resistance as compared with the alternating current impedance method.
Hereinafter, a method of measuring an internal ohmic resistance of a fuel cell by using the current interruption method according to the related art will be described with reference to FIGS. 1 and 2.
Referring to FIG. 1, for measurement of the ohmic resistance through the current interruption method according to the related art, a fuel cell 10 is connected to a main energy consumption device 12 (for example, a driving motor of a fuel cell vehicle, or various electric loads) to apply a current, a separate voltage measurement device 14 is connected to the fuel cell 10, and an interrupter 16 (for example, a switch, a relay, an Insulated Gate Bipolar mode Transistor (IGBT), or the like) for applying or interrupting the current is connected between the fuel cell 10 and the main energy consumption device 12.
Referring to FIG. 2 showing an equivalent circuit of the fuel cell, representing a loss resistance for a voltage of the fuel cell, an activation loss resistance according to a reaction activation of the fuel cell and an internal ohmic resistance of the fuel cell are connected to each other in series, and a capacitor which is an activation loss resistance is connected in parallel to the activation loss resistance according to the reaction activation of the fuel cell.
For measurement of the internal ohmic resistance of the fuel cell while the loss resistances exist, the interrupter between the fuel cell and the main energy consumption device is switched off to interrupt current flowing from the fuel cell to the main energy consumption device.
When the current is instantaneously interrupted as described above, voltage corresponding to the ohmic resistance is instantaneously increased, and voltage corresponding to the activation loss resistance is also slowly increased through a discharge of electric charges stored in the capacitor. At this time, a voltage measurement device measures the voltage.
That is, the voltage measurement device measures the instantaneously increasing voltage Vohmic when the interrupter interrupts the current applied from the fuel cell to the main energy consumption device, and the ohmic internal resistance Rohmic of the fuel cell may be calculated by dividing the voltage Vohmic by the current I which the fuel cell has applied, as indicated by Equation 1.Rohmic=Vohmic/I  Equation 1
The measured ohmic internal resistance is increased as the performance of the fuel cell is reduced by degree of deterioration or water moisture content in the membrane electrolyte assembly is decreased, and thus can be utilized as a barometer to indirectly indicate the deterioration degree and the moisture supply state of the fuel cell.
However, the above-described method of measuring the internal ohmic resistance of the fuel cell according to the related art has following problems.
When measuring the internal ohmic resistance of the fuel cell, current from the fuel cell to the main energy consumption device is interrupted when the interrupter between the fuel cell and the main energy consumption device is switched off, so that operation of the fuel cell system must be stopped.
That is, the fuel cell system cannot be continuously operated since the operation of the fuel cell system is instantaneously stopped for measurement of the internal ohmic resistance of the fuel cell.